First make the buttered leeks and grains. Cut off the very tip of the root and slice the leeks into finger-width pieces, discarding only the toughest green leaf tops.
Warm the butter in a wide pan with a lid over a low heat. Add the leeks and turn them over in the fat before adding ½ teaspoon salt, then cover and cook gently for 20 min.
Add the whole grains together with the white wine and heat through for 5 min. Season with plenty of black pepper, then set aside.
Place the squash on a roasting tray with the sage and tumble around in the olive oil. Roast in the oven for 30 min, or until caramelised and soft.
In a large saucepan, gently reheat the leeks and grains in the stock for 10 min.
Reserve about one third of the squash as a garnish and mash the remainder with the back of a spoon, or blend if you prefer. Stir the mashed squash into the grains, along with a good spoonful of the parmesan. Taste and balance the seasoning with salt, black pepper and lemon juice.
Serve straight away, topped with the remaining squash. Sprinkle with more grated parmesan at the table.
